High voltage electric terminator



May 20, 1958 J. H. NICHOLAS HIGH VOLTAGE ELECTRIC TERMINATOR Filed Feb.1, 1954 CON oucrwt com-ms or TAPERED TmcKnEss comoucrwp GLAZE, 3|

m 9 @w 7 7 5B 7 Z 5 4 I 5 B Raim I u 1 INVENTOR. JAMES H. NICHOLASUnited States Patent D f HIGH VOLTAGE ELECTRIC TERMINATOR James H.Nichoias, Chicago, Ill., assignor to G & W

Electric Specialty Company, Chicago, Ill., a corporation of IllinoisApplication February 1, 1954, Serial No. 407,255

6 Claims. (Cl. 174-442) This invention relates to high voltageelectrical potheads such as are used for terminating the end of aninsulating conductor, particularly a cable conductor, or for terminatinga conductor constituting one terminal of a high voltage transformer orcircuit breaker. The present invention is particularly useful inconnection with potheads or cable terminations at voltages above the 160kv. class.

It is one of the objects of the present invention to provide means forcontrolling the potential gradient in the outer insulator of a potheador terminator structure to maintain as far as possible a uniform axialpotential gradient in the transition from the radial electric field tothe axial or longitudinal field.

The use, on extra high voltages, of potential gradient control meanssuch as have heretofore been employed in potheads up to 160 kv. wouldgenerally result in an extremely large porcelain tube or tubes, whichwould be very costly and ineflicient. A considerably greater length andlarger bore diameter would generally be required. Therefore, the abilityof the porcelain tube to withstand the high internal hydraulic or gaspressures employed on pipe type cable systems would be reduced. Therupture stresses might be so great asto make the design infeasible.

It is one of the objects of the present invention to provide means forcontrollingthe external surface potential gradients in such a manner asto keep them at a more uniform value for the entire overall length ofthe porcelain of the pothead, thus using all of the surface with anequal and therefore high efliciency. In accordance with the principlesof the present invention there is employed a ceramic tube of fairlyrugged wall section and of material having an extremely high dielectricconstant, that is, a specific inductive capacity of 100 to 200. Byproperly applying a conducting media on the external surface of such atube or cylinder 21 very effective radial stress control tube can beobtained. This is so because the radial voltage division between twocoaxial electrodes insulated by two or more dielectric materials is notonly a function of the various radii of the electrodes but also of thedielectric constant or specific inductive capacity (S. I. C.), and thethickness of the various dielectric materials. The higher the S. I. C.of one material used compared to the S. I. C. of the other material theless voltage the higher S. I. C. material has developed across it.Therefore, if the ratio of the S. I. C. of the two materials is madequite high, say 50 or 100 to one, then the high S. I. C. material willhave a very small potential drop across it.

Thus, even though the outside surface of the high S. I. C. stresscontrol tube is metallized, the effect of the voltage division is suchthat the electrode potential would appear to exist at the inner surfaceof the tube because the S. I. C. of the cable and built-up dielectric isroughly 3, compared to 100 to 200 of the new control tube.

The proximity shielding effect is, therefore, not limited bymanufacturing difficulties and fragile construction but is dependentonly on how close radially the stress cone 2,835,725 Patented May 20,1958 shielding braid can be brought to the inner wall (bore) of the highS. I. C. material.

One method of obtaining uniform surface voltage division on alternatingcurrent equipment is by resistance division. By employing severalresistor units in series, and controlling the resistance of each unit toa certain ratio of the total, a definite voltage division can beobtained across each resistor. Ideally, ten resistors of identical valueconnected in series would each have 10% of the total voltage appliedacross each unit. This would then be a perfectly uniform voltagedivision. However, the resistance division, as applied to potheads andbushing, is complicated by the fact that a capacitance exists betweeneach unit resistor and the insulated conductor which passes within theunit resistor. The resistor units near the grounded end of the structurewould be required to carry more capacitance current than those near theconductor or line potential end. This increased current passing throughthe units nearer the grounded end would produce a greater potential dropacross these units than across the unit near the line end. Therefore,the uniform potential gradient would not be obtained.

It is an object of the present invention to use resistor units ofdiminishing values connected in series between the grounded end and thelive end and properly graded so that a uniform voltage division isobtained.

For simplicity in manufacture and production dielectric testing, it isdesirable that individual resistor sections be employed. However, aone-piece construction which results in an electrically equivalentresistance could be used.

It is a still further object of the present invention to provide aresistance assembly for controlling the potential gradient in the outerinsulator of a high voltage terminator, which assembly is of an annularshape so that it can be slipped over the end of the high voltageconductor in assembling the terminator.

It is a still further object of the present invention to provide acombination of resistor units for controlling the stress distributionthroughout the body of the outer insulator and a radial stress controlunit adjacent the grounded end of the terminator, so arranged as toafford the necessary controls wherever the dielectric stress wouldotherwise become excessive.

While the high voltage potential gradient control apparatus of thepresent invention is primarily intended for use on cable systems whichdepend upon high internal hydraulic or gas pressures for theirsatisfactory operation, the principles of the present invention are alsoapplicable to potheads for low pressure systems.

The attainment of the above and further objects of the present inventionwill be apparent from the following specification taken in conjunctionwith the accompanying drawings forming a part thereof.

In the drawings:

Fig. 1 is a longitudinal sectional view of a terminator embodying thepresent invention;

Fig. 2 is a half elevational view and a half longitudinal sectional viewof a resistance unit of the present invention;

Fig. 3 is a plan view of the resistance unit in partial section; and

Fig. 4 is a view similar to Fig. 2 but showing an alternate resistancegrading tube.

Reference may now be had more particularly to the drawings wherein likereference numerals designate like parts throughout.

In the high voltage cable terminator of Fig. 1 a cable 1 is mechanicallyconnected to an outer porcelain insulator 2 of a pothead or terminator 3in the same manner as shown and more fully described in my pendingapplication Serial No. 219,294, filed April 4, 1951, now

3 Patent No. 2,748,184, to which reference may be had. Means for sealingthe end of a cable that enters the pothead may be the same as that shownand described in my application above referred to.

The cable conductor is indicated at 5 and it extends longitudinallythrough the outer porcelain insulator The cable conductor 5 within theinsulator 2 is surrounded by conventional wrapped paper insulation 7 anda stress cone insulator 9 is formed around the wrapped paper insulation7. The stress cone may consist of an impregnated wrapped paper tubetightened on the cable insulation 7 during installation. Surrounding thestress cone insulator 9 is a wrapping of cover insulation 11. Thecovering insulation 11 is preferably a wrapping that can be pressedconsiderably without permanently distorting the same. One suitablematerial is a spongy crepe paper. Surrounding the covering insulation isan as sembly 12 of resistance potential gradient control units whichequalizes the axial potential gradient of the insulator body 2 in themanner to be described.

A grounded stainless steel conical body 13 is secured at its lower endto the terminator mounting plate and is suitably gasketed to provide aliquid-tight seal. A metal ring 15 is welded within the body 13 forsupporting the stress control assembly 12. The ring 15 has a series ofsupporting bolts 17 threaded thereinto which are secured in place bylock nuts. The supporting bolts 17 are uniformly spaced from oneanother, there being any suitable number of such bolts, six, eight, ormore. The supporting bolts 17 are of metal and support at their upperends a base or seating ring 19 of metal, on which seating ring theresistance grading equipment 12 rests. The body 13 has a metal ring 21welded to the top thereof to facilitate securing the outer porcelaininsulator 2 in place as by bolts 23 threaded into a ring 25 cemented tothe bottom of the outer porcelain insulator 2.

The resistance grading equipment 12 of the present invention includes aradial stress unit 26 that rests on the seating ring 19 and in turnsupports a series, in this instance ten, of resistance potentialgradient control units 27. The radial stress unit 26 is a hollowcircular tube or sleeve of fairly rugged wall section, having lower andupper surrounding flanges. The unit 26 closely surrounds the coveringinsulation 11. The material of which the tube is made is preferablyceramic, and of an extremely high dielectric constant (specificinductive capacity between 100 and 200). One suitable material, by wayof example, is titanium dioxide. A conductive glaze or coating is formedon the outer cylindrical portion of the unit 26, covering substantiallythe entire cylindrical surface of the inner sides of the peripheralflanges where the metallic glaze terminates. The top and bottom of theunit are formed as perfectly fiat surfaces parallel to one another andat right angles to the longitudinal axis of the unit.

The flanges of the unit 26 merge with the body of the unit along smoothcurves, free of sharp edges, so that the metal glaze on the outside ofthe unit is also free of sharp edges where it extends from thecylindrical portion to the flanged portion of the unit.

The resistance control units 27 are similar to radial stress unit 26 andclosely surround the wrapping or covering insulation 11. Each of thepotential gradient control units 27 includes a hollow circular tube orsleeve 29 made of the same material as the body of radial stress unit26. A high resistance coating or glaze 31 is formed on the outercylindrical portion of each potential gradient control unit 27 andextends between upper and lower axially spaced flanges 33 and 35 formedin the ceramic tube 29. The high resistance coating 31 coverssubstantially the entire cylindrical surface of each ceramic tube 29 andextends part way along the inner sides of the peripheral flanges 33 and35 where the glaze terminates. The top and bottom of each gradientcontrol unit 27 are 4. formed as perfectly flat surfaces parallel to oneanother and at right angles to the longitudinal axis of the unit.

The high resistance glaze 31 may be made of any suitable well knownresistance conductive glaze materials.

The high resistance coatings on each of the gradient stress controlunits are connected in series with the coating on the units above andbelow it. The outermost units are connected respectively betweenconductor and ground potential. The means by which this is accomplishedwill be explained further on in the specification.

In considering the flow of cur-rent through the connected resistanceelements of the grading tubes there are a number of factors that must beborne in mind. One is the usual current flow through the resistancesthatare connected in series from the line potential to the groundpotential as determined by what might be called the axial resistance ofthe respective units. The other is the radial capacitance currentthrough the resistances due to the capacity existing between eachconductive glaze 31 and the conductor 5. It is apparent that all of theunits are required to carry the resistance current resulting from theaxial series resistance and that the units 27 near the grounded end ofthe structure would be required to carry more of the radial capacitancecurrent than is required of those units that are near the conductor orline potential. This increased current passing through the lower unitswould normally tend to produce a greater potential drop across the lowerunits than the potential drop across the units near to the top of thepothead. Therefore, a uniform potential gradient would not be obtainedif the respective grading tubes 27 having the same axial length wouldhave the same resistance values. By providing the resistor elements withhigh resistance coatings which are graded in value such that theresistances of the resistance units 27 decrease progressively from thehigh potential to the grounded end thereof, a substantially uniformvoltage division between the respective grading tubes may be obtained.For mechanical simplicity all of the resistance grading units 27 may beof a similar overall size and construction. However, as pointed outpreviously, there is an electrical advantage in making these controlunits of different resistances reaching from a minimum resistance valueat the lowermost unit and gradually increasing in value to the uppermostunit. The variation in resistance is obtained by any suitable means, asfor example, by grading the thickness of the high resistance coating oneach ceramic tube 29. Assuming that the conductive coatings are eachmade of identical material, then the grading is obtained by varying thethickness in an amount inversely proportional to the desired resistancevalues.

The specific inductive capacity of the insulating tubes 29 of eachgradient control element 27 is of a much higher value than the specificinductive capacity of the covering insulation 11, the stress controlcone 9, and of the cable insulation 8, which, for example, may be in theneighborhood of four. In such case, substantially all of the voltagedrop between the cable conductor 5 and the coating 31 at any pointappears across the insulation between the tubes 29 and the cableconductor 5. In such case, the potential on the inner surface of therespective ceramic grading tubes 29 is only a fraction of one per centdifferent from the voltage on the outside of the tubes which carries thehigh resistance coatings 31, so that within the pothead insulator thepotential on the outer surface of the wrapped insulation around thecable conductor is, at each point of the axial length thereof, at avalue s'ubstantiallyequal to that of the adjacent outer conductivecoating 31. The resistance grading tubes 27 are arranged to provide auniform step voltage stress distribution of the line voltage at the topof the pot-. head to ground voltage at the bottom thereof and,therefore, substantially the same voltage distribution is obtained alongthe outer surface of the cable conductor covering insulation.

The respective gradient potential control units 27 make a snug fitaround the covering insulation 11 which, due to its compressiblecharacter, provides a suitable medium to take care of any radicalexpansion due to heating of the cable proper. Each axial resistance unit27 is conected electrically in series with its adjacent units. This isaccomplished by respective helically coiled metal garter springs 39which embrace the high resistance coatings 31 at the flanged ends ofeach of the potential gradient control units. The resistance of eachunit is determined by the net resistance of the conductive coatingsextending between the metal springs 39. Each spring is stretched andtherefore tensioned by the cylindrical body of the resistance gradingtube, so that each spring remains in place and in electrical contactwith the end portions of the resistance coatings thereon. The connectionbetween adjacent resistor units is formed by short braided copper bronzejumper leads 41 each of which is soldered or otherwise electricallysecured at its opposite ends to the springs on adjacent resistancegrading tubes, as may be seen from Fig. l. The lowermost grading tube isconnected at its bottom half by a jumper lead 42 to a spring 43,identical with the spring 39, that surrounds and is tensioned around theconductive coating 31 on the stress unit 26, said spring being alsoconnectedbya similar jumper lead 45 to one of the grounded bolts 17. Theuppermost spring of the series of resistance grading tubes is connectedby a jumper lead 47 to a metal yoke 49 that rests upon the upper flangeof the uppermost grading control unit 27. The jumper lead iselectrically connected to the cable conductor at the other end. It isthus apparent that the resistance grading tubes are connected in seriesbetween ground potential at their lower end and the conductor potentialat their upper end. The bight portion 51 of the yoke 49 has a centrallylocated hole therethrough through which a metal connector stud 53extends that is mechanically and electrically secured to the end of thecable conductor 5. The yoke 49 may be pressed downwardly by a suitablecoil spring that bears on the top of the yoke. The yoke 49 thus pressesagainst the top of the uppermost potential grading tube 27 and maintainsall of the grading tubes in engagement with each other and the lowermosttube in engagement with the seating ring 19 of the pothead assembly.

The upper portion of the pothead may be sealed in any conventionalmanner, for instance, as shown in my pending application Serial No.219,294 above referred to, to which reference may be had. The seal isgenerally effected by means of the hood 55 which is secured in sealingengagement with the top of the outer ceramic insulator 2 with the aid ofbolts 57 and a gasket which extends between the hood 55 and the upperneck of the insulator 2.

It is desirable that individual grading tube sections be employedbecause this simplifies manufacturing problems and permits testing ofthe respective units and rejecting those units which do not have thedesired resistance grading without affecting the usefulness of the otherelements. However, a one-piece construction of two or any number, up toall, of the units 27 may be provided. This is illustrated in afragmentary manner in Fig. 4 wherein the unit 60 comprises all of theseparate units shown in the previously described embodiment. Thus asingle cylindrical ceramic tube 62 is provided which has a. resistancecoating 64 on the outer cylindrical surface thereof the thickness ofwhich is graded from one end of the tube to the other so that theresistance of the tube gradually decreases from the high potential endthereof to the grounded end thereof. In the unit of Fig. 4 theresistance may be varied in steps rather than as a continuous gradation.This may be done by coating various adjacent circumferentially extendingareas with different resistivity glazes and allowing them to blend atthe junction between two adjacent circumferentially extending areas or,if desired, two adjacent circumferentially extending bands of differentresistivity glazes may be deliberately separated, which separations areshunted by copper jumpers, etc., for instance, such as the jumpers 41and springs 39, during test and in stallation.

The high voltage pothead described above is primarily adapted for use incable systems which are of the high internal hydraulic or gas pressuretype. The principles can, however, be applied to potheads for lowpressure systems. In the design of such systems there is much greaterleeway as to the outer porcelain bore diameters, since these diametersare not limited by the porcelain rupture stresses involved on the highpressure systems.

In compliance with the requirements of the patent statutes I have hereshown and described a preferred embodiment of my invention. It is,however, to be understood that the invention is not limited to theprecise construction here shown, the same being merely illustrative ofthe principles of the invention. What i consider new and desire tosecure by Letters Patent is:

1. A high voltage terminator or joint structure having a high voltageconductor extending axially therein, and a plurality of resistanceelements spaced axially within the structure and stacked one uponanother and electrically connected in series between the conductor andground, said resistance elements having outer ends which are shaped toabut substantially the entire confronting end faces of the adjacentresistance elements to permit the close stacking of the resistanceelements.

2. A high voltage terminator or joint structure cornprising an outerhollow body of solid insulation into which a conductor extends, meansmaintaining one end of the body at ground potential and the opposite endat the potential of the conductor that extends through the body,covering insulation around the conductor within the bo y, and means forcontrolling the axial potential gradient of the outer hollow insulatingbody, said last means comprising resistance means within the outerhollow body of insulation and extending axially along the coveringinsulation and in circuit between ground and the end of the conductor,said resistance means having axially spaced sections progressivelydecreasing in value axially from the end connected to the conductor tothe opposite grounded end thereof.

3. A joint or terminator structure for a high voltage conductor,comprising an outer insulation into which the conductor extends, meansmaintaining one end of the outer insulator at ground potential and theopposite end at the potential of the conductor that extends through theouter insulator, means for controlling the axial potential gradient inthe outer insulator, said last means comprising axially spacedresistance elements within the hollow insulator and each surrounding andin radial capacity coupling relationship with the conductor, meanselectrically connecting adjacent resistance elements in series betweenthe high voltage conductor and ground, the respective series connectedresistance elements being of progressively different values and of arelationship which is inverse to the relationship of the current flowingthrough them so that the voltages across the respective resistanceelements are the same.

4. A joint or terminator structure for a high voltage conductor, saidjoint comprising an outer hollow insulating body into which theconductor extends, covering insulation around the conductor within thehollow insulating body, means maintaining one end of the hollowinsulating body at ground potential and the opposite end at thepotential of the conductor that extends therethrough, and means forcontrolling the axial potential gradient of the outer insulating body,said last means comprising a sleeve of solid insulation closelysurrounding the covering insulation and location within the hollowinsulating body, said sleeve having a specific inductive capacity whichis many times greater than that of the covering insulation so that thepotentials at the opposite inner and outer surfaces of the sleeve aresubstantially the same, and a layer of high resistance materialcontiguous to and extending axially along the said insulating sleeve,said layer of high resistance material being electrically connectedbetween ground and the end of the conductor and progressively decreasingin value axially from the end of the sleeve connected to the conductorto the opposite grounded end thereof.

5. A high voltage terminator or joint structure having a hollow outerinsulator and a high voltage conductor extending therein, and meansmaintaining one end of the outer insulator at ground potential and theopposite end at the potential of the conductor extending therethrough,means Within the hollow insulator for controlling the voltage gradientof the hollow insulator, comprising a series of high resistance elementssurrounding and aligned axially of and being insulated from theconductor, means electrically connecting the adjacent ends of the highresistance elements to form a series circuit of said elements, and meanselectrically connecting the outer resistance elements respectively tothe said conductor and to ground, said high resistance elements eachincluding a separate, hollow cylindrical insulating member having acylindrical sleeve of a high resistance conductive material coatedthereon.

6. In a high voltage terminator of joint structure having a high voltageconductor extending thereinto, first means for controlling the voltagegradient of the structure comprising a series of high resistanceelements surrounding and aligned axially of and being insulated from theconductor, second means electrically connecting the adjacent ends of thehigh resistance elements to form a series circuit of said elements, andthird means electrically conmeeting the outer resistance elementsrespectively to the said conductor and to ground, said resistanceelements each including a separate, hollow cylindrical insulating memberhaving a cylindrical sleeve of a high resistance conductive materialcoated on a surface thereof, said cylindrical, conductive coatedinsulating membersabutting the adjacent insulating members at the endsthereof, the said conductive coatings thereon terminating short of theends of the members, and said second means comprising respectiveconductive resilient rings tensioned about opposite ends of theconductive coatings on the insulating members, and conductive jumperselectrically connecting adjacent resilient rings.

References Cited in the tile of this patent UNITED STATES PATENTS1,868,962 Atkinson July 26, 1932 1,950,608 Hanson Mar. 13, 19342,386,185 Beaver et al Oct. 9, 1945 2,523,856 Baker Sept. 26, 19502,637,778 Kodama May 5, 1953

